US 7046711 B2
A diode pumped solid state laser for producing a high aspect ratio beam comprises a diode pumping array (1) on a diode array mount (3) and optical means for imaging a pump light beam onto a substantially asymmetrical spot with a smooth intensity profile. The pump light beam is pumping a laser medium (4). Both the pump and the lasing mode have strong asymmetries. In combination with the right choice of laser medium (4), this results in high power laser performance. The axis of the pump light beam is adjustable by a simple adjusting means (110) to a defined plane or direction relative to a mounting frame (111) of a diode array pumping device (103). The adjusting means (110) compensates small tolerances on mounting of the diode array (1) and/or at least one optical element (2). The adjusting means include at least one wedged window (127). Because of this adjustment the axis of the light beam lies in a defined plane relative to the mounting frame (111) of the pumping device (103). Therefore the diode array pumping device (103) of a laser is replaceable without any further adjustment.
1. A laser means for producing an elliptical high aspect ratio spot, comprising:
a diode pumping array with a plurality of emitters, wherein at least two of the emitters, each emitting a partial beam, are mounted in a horizontal array; and
optical means for producing a pump beam by imaging each single emitter into a same spot, wherein said optical means further includes:
an upstream optical means to collimate said partial beam in a vertical plane, and
a downstream optical means to collimate said partial beam in a horizontal plane, focus said partial beam in the vertical plane, and direct said partial beam to said spot, whereby images of said emitters in said spot form a smooth spot by an overlap of said images in a sense that if some of said emitters die or degrade, said spot will not substantially change a spot intensity pattern.
2. A laser means for producing an elliptical high aspect ratio spot, comprising:
a pumping array with a plurality of emitters wherein at least two of the emitters, each emitting a partial beam, are mounted on a horizontal array; and
optical means for producing a pump beam by directing each partial beam to a same spot as a partial beam that is collimated in at least one plane, wherein the optical means further includes:
a first cylindrical lens for collimating the strongly divergent pump light of said partial beam, wherein said first cylindrical lens is positioned nearby said emitters at a distance corresponding to the focal length of the first cylindrical lens; and
a first lens for collimating said partial beam in a horizontal plane and focusing said partial beam in a vertical plane and directing said partial beam to said spot, wherein said first lens is positioned at a distance away from the diode pumping array corresponding to the focal length of the first lens.
3. A laser means for producing an elliptical high aspect ratio spot, comprising:
a diode pumping array with a plurality of emitters, wherein at least two of the emitters, each emitting a partial beam, are mounted in a horizontal array; and
optical means for producing a pump beam by directing each partial beam to a same spot as a partial beam that is collimated in at least one plane, wherein said optical means includes:
a first cylindrical lens for collimating a strongly divergent emission of an emitter into a beam in a first plane, wherein said first cylindrical lens is positioned near the diode array at a distance corresponding to the focal length of the first cylindrical lens, and
a second cylindrical lens for collimating said beam wherein said second cylindrical lens is positioned at a distance from the diode pumping array corresponding to the focal length of the second cylindrical lens.
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15. The diode-pumped laser operating in the fundamental mode, comprising:
a laser means according to
a solid state laser medium which is excited by said laser means.
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27. A solid state laser medium excited by a laser means according to
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31. A diode-pumped laser operating in a fundamental mode, comprising:
a laser means for producing a high aspect ratio beam comprising a diode pumping array and optical means for imaging a pump light beam into a substantially asymmetrical spot with a smooth intensity profile; and
a laser medium which is excited by said pump light beam, wherein an axis of the pump light beam is positioned at least one of obliquely and vertically to an axis of the fundamental mode of the laser medium, wherein the laser medium is bonded to a heat sink on a bottom side of the laser medium, wherein the pump light beam is incident on a top side of the laser medium, and wherein the fundamental mode of the laser medium is operated perpendicular to a heat flow in the laser medium.
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This is a Continuation-in-Part of Application No. PCT/EP00/05336 filed Jun. 9, 2000, which in turn claims priority from 60/138,906, 60/146,472 and 09/489,964. The entire disclosure of the prior application(s) is hereby incorporated by reference herein in its entirety.
This invention relates to the field of compact, laser-diode pumped solid state laser sources using a scheme where both the pump and the lasing mode have strong asymmetries.
High power diode-pumped lasers have been a subject of intense research recently. Powers in the range of several 100 Watts have been demonstrated from such laser systems. However, with respect to fundamental mode operation (TEM00-mode), such diode-pumped laser sources have shown only a few tens of Watts of output power even in various approaches to the matter. These TEM00, high power diode-pumped laser sources are typically based on the laser mediums Nd:YAG, Nd:Vanadate, Nd:YLF or Yb:YAG. However, in the case of generating TEM00 modelocked output (picosecond or femtosecond pulses) or TEM00 frequency doubled output (green at a wavelength of 532 nm from diode-pumped Nd:Vanadate, for example), even in relatively complicated setups lower powers of the order of 10 Watts or less have been produced. Among the challenges are: keeping the overall setup simple and compact; in modelocked systems: keeping the pulse width as short as possible for subsequent nonlinear optical applications (<10 ps for Nd:Vanadate, for example), in modelocked systems using semiconductor saturable absorbers: finding a design which puts the laser safely into stable modelocking sufficiently far above the modelocked-Q-switching threshold, in intracavity frequency converted laser systems: avoiding the “green problem” or similar output instabilities.
It has long been thought that intracavity optical frequency doubling is subject to the “green problem”, which results in unstable fluctuations of the frequency-converted output from the laser. Recently some setups have been disclosed which overcame this problem. This invention shows another method to avoid the green problem.
Modelocking of solid state lasers has been achieved with semiconductor saturable absorbers. However, the dynamics of the modelocking calls for high gain saturation, which can be achieved with the schemes described here. The higher the gain saturation, the lower the pulse energy density on the semiconductor saturable absorber can be reducing the degradation rate of the semiconductor saturable absorber. High gain saturation is usually obtained by achieving high pump intensity, which is however limited by the fracture limit of laser materials. Therefore, pump powers in these types of lasers have typically been limited to 10–20 W pump power per spot in state-of-the-art Nd:Vanadate lasers, or multiple pump spots have been used, adding complexity to the system. Furthermore, thermal lensing limits the maximum pump power per spot.
The invention relates to the field of compact, laser-diode pumped solid state laser sources using a scheme where both the pump and the lasing mode have strong asymmetries. A substantially smooth asymmetric laser diode pump spot is obtained from a laser diode array source or multiple arrays, by imaging each single emitter of the array or the arrays into substantially the same spot at the laser medium. In combination with the right choice of laser medium, this results in high power laser performance. This can also give rise to high gain saturation inside the laser medium. As an alternative, a diode pumping setup and crystal geometry where the pump beam has not the same propagation axis as the laser mode can be used. The laser medium does not necessarily have a coating reflective for the laser wavelength attached to it. In combination with an intracavity nonlinear optical element, such as a nonlinear optical crystal or a semiconductor saturable absorber, this can lead to stable intracavity nonlinear conversion and stable ultra short pulse generation, respectively. A special semiconductor saturable absorber structure for generating ultra short laser pulses is described. Additionally, external frequency conversion schemes can be efficiently used to generate other frequencies from the ultra short pulse output. The scheme may, however, also be used for high power continuous-wave or Q-switched laser operation. According to a special embodiment of the invention, The axis of the pump light beam is adjustable by a simple adjusting means to a defined direction relative to a mounting frame of a diode array pumping device. The adjusting means compensates small tolerances on mounting of the diode array and/or at least one optical element. The adjusting means include at least one wedged window. Because of this adjustment the axis of the light beam lies in a defined plane relative to the mounting frame of the pumping device. Therefore the diode array pumping device of a laser is replaceable without any further adjustment.
An additional field of application is the pumping of a laser source containing a regenerative amplifier or other single- or multi-pass amplifier setups. For this type of setup small and focused pump laser spots lead to problems as optical damages and as a too high gain may occur within a limited volume of the laser medium, which may lead to beam distortion and diffraction losses. A pumping device which produces a pattern-free profile with reduced intensity fulfills the special requirements of this setup. Suitable pump spots have a cross section with a substantially low aspect ratio, that for example is 1:1 or may be defined by the refraction index n as 1:n (e.g. with n=2,16 for Nd:vanadate).
The invention is not restricted to the combination of the described parts of the laser, but also to the parts as such.
High intensity pump:
The laser diode used preferably emits from an emitting area which is 1 micrometer (μm; micron) in height (vertical dimension) and typically 2 mm up to 10 mm (horizontal dimension). Laser diodes with a 10 mm wide emitting area on a single chip emit output powers in the 20–60 W range in a highly non-diffraction limited beam (ca. 2000 times the diffraction limit in the horizontal dimension for a 10 mm wide emitting area). These high power diode “bars” usually consist of an array of lasers provided on the same chip: For example, a 40 W laser diode may consist of an array of 20 lasers each with a 200 μm wide stripe and a center-to-center spacing of 400 μm. In this case, the fill factor is 50%, corresponding to the active area divided by the total area, which also includes the inactive spacer area in between two adjacent diodes on the same chip or “bar”.
For the purpose of focusing the diode laser light into a laser medium 4, a f=8 mm focal length lens 5 with a high NA and good imaging quality is used, for example the item number C240TM-B from Thorlabs, Inc. This lens 5 is positioned at a distance of about 20 mm from the diode laser 1. Aspherical lenses can have the advantage of improved imaging quality and therefore higher intensity is achieved at the pump spot.
The focusing optics as described generates a bright and highly asymmetric pump beam inside the laser medium 4, which is well suited for pumping a laser cavity with a vertically squeezed laser mode inside the laser medium 4. This is described in more detail in D. Kopf, K. J. Weingarten, G. Zhang, M. Moser, A. Prasad, M. A. Emanuel, R. J. Beach, J. A. Skidmore, U. Keller, Invited Paper, “High-average-power diode-pumped femtosecond Cr:LiSAF lasers,” Applied Physics B, vol. 65, pp. 235–243, 1997. The invention uses a different concept based on different focusing and on different laser gain materials. Furthermore, different crystal and pumping geometries are described.
The use of a high NA focusing lens, as mentioned above, has the advantage that a high intensity spot is generated. According to the setup described above, a pump spot is generated which is 740 μm×20 μm (radius) in the horizontal and vertical plane, respectively. At a power of 15 Watts, this corresponds to a pump density of 32 kW/cm2. With higher powers and equivalent beam parameters (divergence, stripe width) the pump intensity would go up correspondingly. This corresponds to pump densities which are hardly achieved otherwise in such a simple manner. Based on the concept described above, a man skilled in the art can devise with similar and modified setups which also achieve a high intensity spot.
The distance between the laser diode array 1 and the focusing lens 5 can be changed which results in an astigmatism after the focusing lens, i.e. the pump beam waists in the horizontal and vertical dimension, respectively, are at different locations. The astigmatism can be used to generate a tailored and predetermined ellipticity of the pump beam at the entrance face of the laser medium 4, resulting in tailored thermal lenses in the two transverse dimensions.
Likewise, the distance between the laser diode array 1 and the focusing lens 5 can be changed in order to control the intensity distribution at the laser medium 4. It is usually preferable to achieve an as smooth as possible transverse power distribution with a negligible pattern to it. This can be accomplished if the image of the laser diode array 1 is as far away from the focal plane of the lens as possible, ideally infinitely far away. This is achieved if the laser diode array 1 is positioned one focal length away from the lens 5, which would be at a distance of 8 mm in our case. Satisfactory results, however, may also be obtained in the vicinity of this condition. Applied to our setup, a substantially smooth light distribution at the pump spot is obtained if the f=8 mm focusing lens is positioned 8 mm beyond the laser diode array 1, with a fast axis collimating cylindrical lens 2 positioned next to the latter. A smooth light distribution results in a smooth an pattern-free thermal distribution and therefore in a thermal lens which facilitates or enables laser operation in the fundamental mode.
The laser diode array 1 is mounted on a submount 3 which is positioned on a laser system base 28. The laser diode array 1 may be constructed as a module within a housing. In this case a wedged window 27 is provided for the pump beam to pass.
Choice of laser gain material, crystal geometry, pumping geometry, cooling: The highly assymetric, essentially elliptical pump spot obtained as above is focused into the laser gain material, where a great deal of the power is absorbed within the absorption length.
The highly elliptical pump spot also has the advantage of reduced thermally induced birefringence, which was shown to limit laser performance in various lasers, in particular in Nd:YAG lasers. Therefore, the setups described here can also be used to avoid thermal birefringence limitations.
A highly asymmetric pump spot 6 b can also be obtained by choosing a horizontal pump spot size which is equal or almost equal to the horizontal dimension of the laser medium 4 (
As an alternative approach,
The position of the laser medium 4 can be adjusted: The distance from the focusing lens 5 determines the actual pump spot at the entrance face 4′ of the laser medium thus affecting both the laser gain and the thermal lens. This can be used to optimize laser performance, i.e. output power.
The laser medium crystallographic orientation can help reduce the horizontal thermal lens if the laser medium exhibits different thermal conductivity along the crystallographic axes. If the vertical axis is chosen to be the one with the highest heat conductivity, then again the vertical, one-dimensional heat flow is more pronounced, resulting in another reduction in the horizontal thermal lens, thereby forming a basis for further power optimization. This is the case, for example, for Cr:LiCAF, which exhibits a 10% difference in thermal conductivity for the heat flow along the two crystallographic axes.
The polarization of the cavity mode can be chosen such that it points along the crystallographic axis which has lower dn/dT, resulting in lower thermal lensing. This is, for example, interesting when using Nd:YLF, which exhibits less than half the dn/dT for a polarization perpendicular to the c-axis compared to the direction parallel to the c-axis.
Laser medium arrangement and positioning:
Another approach is possible which has the same advantages as end-pumping, but avoids direct end-pumping. Such an approach is shown in
Choice of laser medium: The laser media 4 that can preferably be used for high power laser output are among, but not limited to, Nd:YAG, Nd:Vanadate, and Nd:YLF. These materials have different gain and thermal lensing properties. Among these Nd:vanadate has the highest stimulated emission cross-section and the shortest upperstate lifetime which makes it suitable for stable modelocking and intracavity nonlinear conversion. Nd:YLF has the widest emission band width and can therefore provide the shortest modelocked pulses, for example 3 ps pulses. In addition, Nd:YLF has the lowest thermal lensing which makes it suitable for high power, i.e. >5 Watt output using the schemes of this invention. All of these materials may be used for high laser power performance up to 10 Watts or multiple 10 Watts. A man skilled in the art can optimize the pump beam spot inside the laser medium and the laser mode, aiming at optimized mode matching, results in fundamental mode operation at these power levels. Further materials that can be used in the configuration of this invention include Cr:LiSAF, Cr:LiCAF, Cr:LiSGAF, Cr:forsterite, Cr(4+):YAG, Nd:glass, Yb:glass, Yb:YAG, Er:Yb:glass, Nd:vanadate and Nd:YAG operated at the 900-nm and 1300-nm lines, respectively. Furthermore, dye laser media can be provided as laser media.
The Nd:YLF is particularly suitable for scaling to higher powers and higher peak powers due to its broader emission band (compared to Nd:YAG and Nd:vanadate), its reduced thermal lensing and thermal birefringence effects. Its fragility can be addressed by choosing a sufficiently long absorption length of >2 mm. The high intensity pump described here results in sufficiently high gain saturation required for stable modelocking above the modelocked Q-switching threshold, following the rules of C. Hönninger, et al., in “Q-switching stability limits of continuous-wave passive mode locking”, Journal of Optical Society of America B, Vol. 16, No. 1, pp. 46 ff.
Laser pump setups with a comparatively less small signal gain are suitable for an use within a regenerative amplifier setup. The according minimal amplification of a few percent up to a factor not much in excess of 4 (i.e. 400%) for one roundtrip is sufficient for this type of setup. Note that this is on the order of 10× less small signal gain than achieved in the above-mentioned setup with an asymmetrical pump spot. The characteristic feature is the product of sigma as the emission cross section and tau as the life time of the upper niveau. This product is equal to the small signal gain coefficient. Appropriate values are of the magnitude of 5·10−24 sec/cm2, the value for Nd:YLF or ND:YAG is appr. 6·10−23 sec/cm2, the value for Nd:vanadate is four times higher. These small signal coefficients result in a small signal gain within the range from a few percent per roundtrip up to a factor of ˜4 when the pumping scheme of
Suitable materials are for example: Nd:YAG, Nd:vanadate (Nd:YVO4), Nd:YLF, Yb:YAG, Yb:KYW, Yb:KGW, Yb:SrYBO. Depending on the material-given spectral bandwidth, femtosecond or picosecond pulse amplification is supported.
Intracavity and extracavity frequency conversion: Frequency conversion can be used to generate other wavelengths from a fundamental wavelength, such as 1064 nm in the case of Nd:vanadate and Nd:YAG, or 1053 nm or 1046 nm in the case of Nd:YLF as the laser medium. Frequency conversion systems have been shown to also generate the colors red, reen, and blue from such an infrared picosecond pulse source by using optical parametric oscillators, optical parametric amplifiers, or optical parametric generation. These colors, red, green,and blue, can be used for example in laser projection display applications and for entertainment purposes. Frequency conversion to the ultra violet and to the visible wavelength range results in applications in various fields of research and sciences, such as fluorescence measurements, spectroscopy and confocal microscopy. Frequency conversion to the mid-infrared wavelength range can also be accomplished by employing of optical parametric oscillators and is useful for many applications such as gas sensing and molecular spectroscopy. The laser lines around 900 nm and 1300 nm of Nd:Vanadate (917 nm, 1340 nm), Nd:YAG (946 nm), and Nd:YLF, respectively may also be used as the fundamental wavelength to be converted to other wavelengths. For example, the Nd:Vanadate can be doubled to 457 nm, both in an intracavity or extracavity second harmonic generation scheme.
Intracavity nonlinear conversion can be obtained by placing a nonlinear optical material 30 (according to
The high peak power modelocked output of a laser as described here can be used for external, i.e. extracavity, frequency conversion. Second harmonic generation is accomplished by using a nonlinear optical crystal with known appropriate cut for phase matching at the fundamental and the doubled optical frequency, respectively. Well known materials for this purpose are KTP and its isomorphes, BBO, KNbO3, LBO, LiNbO3, periodically poled LiNbO3, periodically poled KTP and its isomorphes. A laser source with a peak power of 7 kW, such as the one described here, can be converted to the second harmonic with high efficiencies in KTP, provided that phase matching is obtained and the KTP crystal length is optimized according to the rules well known in nonlinear optics. Third harmonic conversion can be obtained by frequency mixing the fundamental and the second harmonic light in another nonlinear optical crystal suitable for third harmonic generation, such as LBO or BBO. Fourth harmonic generation can be obtained by applying the second harmonic generation process to the second harmonic output, for example using BBO to double the frequency of the frequency doubled Nd:vanadate laser. Due to the high peak power of such a picosecond laser system, other nonlinear optical conversion schemes well known in nonlinear optics may be used to generate other frequencies, such as sum frequency generation, difference frequency generation, third harmonic generation, fourth harmonic generation, optical parametric oscillation, optical parametric amplification, optical parametric generation.
Enhanced nonlinear-optical conversion with picosecond pulses can be obtained if the nonlinear-optical material is configured in an etalon-like configuration for resonating each picosecond pulse and thereby enhancing the peak power. For this purpose, the resonator will have to be shorter than the physical length of an optical picosecond pulse.
When the high power picosecond laser is used to pump an optical parametric oscillator cavity, stable synchronisation of both the pump laser cavity and the optical parametric cavity can be obtained, largely independent of thermal drift, by putting both cavities on the same base 28 (
Modelocking mechanisms: The modelocking can be obtained by using a modelocking apparatus inside the laser cavity. Such a modelocking apparatus can be a semiconductor saturable absorber or a multiple quantum well device. Any optical scheme that acts as a saturable absorber can be used as well. The modelocking dynamics using semiconductor saturable absorbers is dependent on the pulse energy density both on the saturable absorber and inside the laser medium, as well as on the saturation parameters of the saturable absorber and the laser medium. While minimizing the pulse energy density on the saturable absorber, aimed at longer term operation and lower degradation rates, stable modelocking remains, provided that the pulse energy density inside the laser medium is increased. With the scheme according to this invention, the pulse energy density inside the laser medium is already made considerably higher than in most continuous-wave diode-pumped solid state Nd:YAG or Nd:Vanadate lasers. Therefore, the scheme is well suited for long term and stable mode locking operation using a semiconductor saturable absorber.
As an alternatative modelocking mechanism, a nonlinear mirror modelocking technique can be used. Such a mechanism is described, for example, in U.S. Pat. No 4,914,658: Modelocked laser, Stankov, et al.
Q-switching can be obtained by incorporating an optical switching device into the laser cavity.
Transversely pumped, transversely cooled small laser medium laser: Another embodiment according to the invention comprises a transversely pumped, transversely cooled, thin laser medium 4 configuration for high power diode-pumped laser operation. A schematic is depicted in
If further the laser pump beam is substantially homogeneous throughout the laser medium volume, the heat flow will result in a substantially parabolic temperature increase along the vertical dimension V through the laser medium 4. Such a parabolic vertical temperature curve results in a parabolic and therefore substantially aberration-free vertical thermal lens, which has great advantages for the power scaling towards higher powers. A mounting and pumping concept similar to the one previously called the “thin disc laser” (A. Giessen, Stuttgart) may be used. The thin disc laser by A. Giessen achieves high pump densities in a thin disc of Nd:Vanadate or Yb:YAG, for example. The difference to that concept is that according to the invention the laser mode is operated transversely to the heat flow rather than parallel to the vertical dimension. If a (vertically) very thin laser medium is used, this results in an even smaller laser mode cross-section and therefore in higher small signal gain compared to the previous thin disc setup. The higher small signal gain can be used to build around this gain medium any kind of modelocking device or frequency conversion device as described throughout this invention. For example, stably modelocked lasers can be built using semiconductor saturable absorber mirrors or stably intracavity-converted continuous-wave lasers.
The diode laser array 1 a according to
The terms “horizontal” and “vertical” are defined by the specific setup of the diode array. In these examples the diode emitters are positioned horizontally in a linear array. Therefore the resulting beam has a cross section with an essentially horizontal scope. The examples do not limit the positioning of the emitters or the cross section of the beams to a horizontal setup. For a different orientation of the emitters, e.g. in a vertical linear array, the terms have to be adapted in an appropriate manner.
An alternative imaging setup is shown in
The U.S. Pat. No. 5,541,951 discloses a device and method for high-power end pumping of a regenerative amplifier setup. The problems related to the high intensity spot are reduced by a pumping device, which is pulse activated for only approximately four hundred microseconds during each millisecond of operation. A disadvantage of this solution is the complexity of pulse generation and the still high intensity within the pump spot even if this intensity lasts only for a short period of time.
The pumping device of
The pump spot pumps a laser crystal which by way of example can be a flat-Brewster-cut crystal, which is coated on the flat side (entrance face for pump light) for high reflectivity at the laser mode wavelength and for high transmission at the pump wavelength, and uncoated or antireflection coated for protection on the Brewster face.
By expanding the laser resonator of
As an example: A 0.3%-doped Nd:Vanadate crystal, mounted according to
As an alternative setup,
Beam axis adjustment for the light beam emerging from the diode array: The imaging optics of known laser means as for example described in ,1.1-W cw Cr:LiSAF laser pumped by a 1-cm diode array″, D. Kopf, U. Keller, M. A. Emanuel, R. J. Beach and J. A. Skidmore; Optics Letters, Jan. 15, 1997, Vol. 22, No. 2 comprise several optical elements, which have to be mounted at precise positions along the path of the light beam from the diode array to the laser medium. These optics extend over a long distance and do need a big space. The laser diode usually emits from an emitting area which is 1 μm (micrometer) in vertical direction and typically 3 μm up to 10'000 μm in horizontal direction. Laser diodes with an emitting area which is 3 μm wide typically emit laser light which is diffraction limited and carries approximately 50–300 mW power. Laser diodes with a 10'000 μm wide emitting area on a single chip emit laser light with output powers in the range of 20–60 W. The laser light is in the vertical dimension nearly diffraction limited but is not diffraction limited in the horizontal dimension (approximately 2000 times higher). The high power diode “bars” usually consist of an array of diodes located on the same chip: For example, a 40 W laser diode may consist of an array of 20 diodes each with a 200 μm wide stripe and a center-to-center spacing of 400 μm. In this case the fill factor is 50% corresponding to the active area divided by the total area, which also includes the inactive spacer area in between two adjacent diodes on the same chip or “bar”.
The light from the laser diode is typically strongly diverging in the vertical axis (“fast” axis) at an angle of approximately +/−30°, for example. Therefore, the first optics component in the imaging optics is a fast-axis collimation lens, preferably a cylindrical lens, which collects the light in the vertical dimension. This lens is normally attached to the same mount on which also the laser diode chip (“bar”) is mounted. In some cases, it is not necessary to use a cylindrical lens, especially as long as the emitting area aspect ratio is not much higher than 500 μm: 1 μm (horizontal dimension:vertical dimension). Generally it is preferable to use high numerical aperture (NA=0.5 approximately) lenses which are capable of collecting all or substantially all power in the vertical dimension, where the divergence is generally much higher than in the other dimension.
One issue of importance is the so-called “smile” of a laser diode array. The smile relates to a bending of the laser diode array in the vertical dimension. This bending is typically in the order of one or a few microns across a 5'000 μm or 10'000 μm diode bar. The result is that the discrete emitting areas on the same chip have different vertical positions with respect to the cylindrical fast-axis collimation (FAC) lens. For a change in vertical position in the order of 1 μm across the bar, the beam angle after the collimation lens is changed by 1/200=5 milli radians. This is approximately equal to the full-angle divergence of the diffraction-limited beam exiting the FAC lens, which is the divergence that would be expected with no smile and no aberrations from the FAC lens. Therefore, any smile greater than 1 μm across the bar would be the predominant reason for increased beam divergence and, correspondingly, reduced beam quality after the FAC lens, and is therefore to be avoided as much as possible.
Likewise, any misalignment of the FAC lens on the order of 1 μm in the vertical dimension results in the same angle deviation. Therefore, the FAC lens would have to be mounted with a tolerance of less than 1 μm. This is difficult and/or costly. To reduce this cost and to make the mounting procedure of the FAC lens to the diode mount less critical, any beam angle tolerances, respectively orientation tolerance of the beam axis, after the FAC lens in the order of +/−10 milli radians in the vertical dimension are acceptable. This leaves the FAC mounting procedure with an acceptable tolerance of +/−2 μm for the vertical position
A preferred configuration should enable a precisely oriented diode array beam by simple optics. The described mounting procedure tolerance should not cause an unacceptable deviation of the diode array beam form its precise orientation.
The vertical angle compensation, respectively adjusting the beam axis into a horizontal plane, results in a change in angle in the horizontal plane. However, this does not result in a significant relative change of the horizontal angle because the divergence of the laser diode light in the horizontal plane is on the order of +/−6° (=ca. +/−100 milli radians). Therefore, a change in angle in the horizontal plane can be neglected.
The vertical beam offset which comes along with the vertical angle compensation can be compensated for by another rotation of the same or an additional window with a rotation axis aslant to the beam axis. This is shown in
At least one wedged window 127 is part of an adjusting means which enables adjusting the light beam, respectively its axis, to a defined plane or axis relative to the diode array mount 3. According to
The adjusting means 110 is very simple and enables a precise orientation of the diode array beam by a an easy adjusting step at the production of the diode array pumping device 103. The diode array pumping device 103 and the adjusting means 110 is new and inventive independent of the other scopes of this invention.
The optics on the diode array pumping device 103 is designed such that the connecting of the pumping device mounting frame 111 to the laser system base 28 is insensitive to angle and position deviation that may occur from one diode array pumping device 103 to the other diode array pumping device 103 upon replacement. Typical repositioning tolerances of the diode array pumping device 103 on the laser system base 28 are in the order of a few microns, corresponding to the achievable flatness in the order of a few microns for machined surfaces. This results in well acceptable changes in beam direction and offset at the laser medium and therefore does not significantly affect the laser system performance.
For the purpose of focusing the diode laser light into the laser medium, an f=8 mm focal length lens 5 with high NA and good imaging quality, for example the item number C240TM-B from Thorlabs, Inc., can be used. This lens 5 is preferably no longer part of the diode array pumping device 103, but is attached to the laser system base 28 where also the laser medium 4 is positioned.
The focusing optics described here generates a bright and highly asymmetric pump beam inside the laser medium, as can be seen from the beam propagation plot in
At the laser medium, one can assume an acceptable relative tolerance of +/−10% for both the diode laser beam angle and position with respect to the diode laser beam divergence and width. Given a half-angle vertical beam divergence of 10 milli radians and a beam waist of 20 microns radius at the laser medium, this corresponds to a desired pump beam reproducibility of +/−1 milli radian in angle and +/−2 microns in position at the laser medium. In a distance of 8 mm (focal length) before the f=8 mm focusing lens, any angle deviation results in a position deviation at the laser medium, and vice versa. Therefore, the requested beam angle and position reproducibility at the diode array pumping device 103 exit is calculated to be 8 μm and 0.25 milli radians, respectively. For typical machining accuracies in the order of a few microns this should be easy to accomplish: Given a surface flatness of 5 μm across the diode array pumping device 103 base dimension of approximately 60 mm (
The example given here is not restricted to the given values. For an ordinarily skilled person it would be obvious how to adapt the set-up described here to different focal lengths, dimensions, number and types of optics elements, etc. For example, the fast-axis collimation lens 2, which had a focal length of 200 μm in this description, can have a different focal length, too. For this purpose, fast-axis collimation lenses with a focal length in the range from 50 μm to 1000 μm are common and often used, but the scope of this invention is not restricted to these values. The fast-axis collimation lens 2 does not necessarily have to be mounted on the same mount as the diode bar is mounted on, but may as well be fixed to the pumping device mounting frame 111.
Likewise, the beam path may contain another lens or focusing means, such as for example a second cylindrical lens for shaping the beam in the horizontal plane. The wedged window 127 does not necessarily have to have flat surfaces but can fulfil substantially the same function when its surfaces are curved. Note that, for example, an off-axis spherical or cylindrical lens could be used for basically the same purpose as the wedged window 127, since the front and back surface are also at an angle with respect to each other. As an alternative, the wedged window 127 can also be replaced by prism which fulfils still substantially the same function as the wedged window because its surfaces are also at an angle with respect to each other.
The diode array pumping device 103 is a module that can be replaced without affecting the laser performance. Such a user-replaceable diode module would increase the laser life time of diode-pumped lasers because the user can replace the diode module himself and therefore no service personnel is needed. Since typical laser diodes only live for approximately 10'000 hours (mean time to failure), the user-replaceable diode module boosts conveniently the laser system lifetime up to multiple laser diode lives, which is sufficient for industrial laser applications. Likewise, solid state lasers which use less reliable laser diodes, for example high power red diodes with a live time of about 2.000 hours, can still become attractive when using the possibility of easy diode module replacement.
Up to now diode-pumped solid state lasers do not use any concept which aims at easy replacement of the laser diodes. Here, the disadvantage is that only skilled personnel at the laser factory can do the replacement. Other laser manufacturers have focussed on fiber-delivery of the laser diode light from diode modules, which then can easily be replaced by an experienced engineer (“field-replaceable”). However, this approach suffers from loss of brightness and beam quality through the optical fiber, from higher complexity and an increased number of optical elements. Additionally, the field-replaceable diode modules do not yet fulfil the criterion of being user replaceable.
It is therefore an important advantage of the described embodiment with a replaceable diode array pumping device 103, that the laser diode and part of the imaging optics is put on a removable optics basis for quick, simple, and reproducible replacement. The optics on the diode module is designed such that after replacement of that module, both the angle sensitivity and the positioning sensitivity is low enough to make sure that the laser still works. The diode module optics contains at least one piece of optics which allows very easy (i.e. insensitive) adjustments of the output beam characteristics, which makes the diode module assembly procedure insensitive and reproducible.